Fin field effect transistor (finfet) device and method for forming the same

ABSTRACT

A fin field effect transistor (FinFET) device structure and method for forming FinFET device structure are provided. The FinFET structure includes a substrate and an isolation structure formed on the substrate. The FinFET structure also includes a fin structure extending above the substrate, and the fin structure is embedded in the isolation structure. The FinFET structure further includes an epitaxial structure formed on the fin structure, the epitaxial structure has a pentagon-like shape, and an interface between the epitaxial structure and the fin structure is lower than a top surface of the isolation structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-pending an commonly assigned patent applications: U.S. Ser. No. 14/______, filed on Oct. 17, 2014 and entitled “ Fin field effect transistor (FinFET) device and method for forming the same” (Applicant Docket No. TSMC 2014-0686; Attorney Docket No. 0941-3065PUS1).

BACKGROUND

Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductive layers of material over a semiconductor substrate, and patterning the various material layers using lithography to form circuit components and elements thereon. Many integrated circuits are typically manufactured on a single semiconductor wafer, and individual dies on the wafer are singulated by sawing between the integrated circuits along a scribe line. The individual dies are typically packaged separately, in multi-chip modules, or in other types of packaging, for example.

As the semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design issues have resulted in the development of three-dimensional designs, such as the fin field effect transistor (FinFET). FinFETs are fabricated with a thin vertical “fin” (or fin structure) extending from a substrate. The channel of the FinFET is formed in this vertical fin. A gate is provided over the fin. Advantages of the FinFET may include reducing the short channel effect and higher current flow.

Although existing FinFET devices and methods of fabricating FinFET devices have been generally adequate for their intended purpose, they have not been entirely satisfactory in all aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 shows a perspective representation of a fin field effect transistor (FinFET) device structure, in accordance with some embodiments of the disclosure.

FIGS. 2A-2F show side views of various stages of forming a fin field effect transistor (FinFET) device structure, in accordance with some embodiments of the disclosure.

FIG. 3 is an enlarged representation of region A of FIG. 2F, in accordance with some embodiments of the disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Some variations of the embodiments are described. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements. It is understood that additional operations can be provided before, during, and after the method, and some of the operations described can be replaced or eliminated for other embodiments of the method.

Embodiments for forming a fin field effect transistor (FinFET) device structure are provided. FIG. 1 shows a perspective representation of a fin field effect transistor (FinFET) device structure 10, in accordance with some embodiments of the disclosure. The FinFET device structure 10 includes a N-type FinFET device structure (NMOS) 15 and a P-type FinFET device structure (PMOS) 25.

The FinFET device structure 10 includes a substrate 102. The substrate 102 may be made of silicon or other semiconductor materials. Alternatively or additionally, the substrate 102 may include other elementary semiconductor materials such as germanium. In some embodiments, the substrate 102 is made of a compound semiconductor such as silicon carbide (SiC), gallium arsenic (GaAs), indium arsenide (InAs), or indium phosphide (InP). In some embodiments, the substrate 102 is made of an alloy semiconductor such as silicon germanium (SiGe), silicon germanium carbide (SiGeC), gallium arsenic phosphide (GaAsP), or gallium indium phosphide (GaInP). In some embodiments, the substrate 102 includes an epitaxial layer. For example, the substrate 102 has an epitaxial layer overlying a bulk semiconductor.

The FinFET device structure 10 also includes one or more fin structure 104 (e.g., Si fins) that extend from the substrate 102. The fin structure 104 may optionally include germanium (Ge). The fin structure 104 may be formed by using suitable processes such as photolithography and etching processes. In some embodiments, the fin structure 104 is etched from substrate 102 using dry etch or plasma processes.

In some other embodiments, the fin structure 104 can be formed by a double-patterning lithography (DPL) process. DPL process is a method of constructing a pattern on a substrate by dividing the pattern into two interleaved patterns. DPL process allows enhanced feature (e.g., fin) density.

An isolation structure 108, such as a shallow trench isolation (STI) structure, is formed to surround the fin structure 104. In some embodiments, a lower portion of the fin structure 104 is surrounded by the isolation structure 108, and an upper portion of the fin structure 104 protrudes from the isolation structure 108, as shown in FIG. 1. In other words, a portion of the fin structure 104 is embedded in the isolation structure 108. The isolation structure 108 prevents electrical interference or crosstalk.

The FinFET device structure 10 further includes a gate stack structure including a gate electrode 110 and a gate dielectric layer (not shown). The gate stack structure is formed over a central portion of the fin structure 104. In some other embodiments, multiple gate stack structures are formed over the fin structure 104.

In some other embodiments, the gate stack structure is a dummy gate stack and is replaced later by a metal gate (MG) after high thermal budget processes are performed.

The Gate dielectric layer (not shown) may include dielectric materials, such as silicon oxide, silicon nitride, silicon oxynitride, dielectric material(s) with high dielectric constant (high-k), or combinations thereof. Examples of high-k dielectric materials include hafnium oxide, zirconium oxide, aluminum oxide, hafnium dioxide-alumina alloy, hafnium silicon oxide, hafnium silicon oxynitride, hafnium tantalum oxide, hafnium titanium oxide, hafnium zirconium oxide, the like, or combinations thereof.

The gate electrode 110 may include polysilicon or metal. Metal includes tantalum nitride (TaN), nickel silicon (NiSi), cobalt silicon (CoSi), molybdenum (Mo), copper (Cu), tungsten (W), aluminum (Al), cobalt (Co), zirconium (Zr), platinum (Pt), or other applicable materials. Gate electrode 110 may be formed in a gate last process (or gate replacement process). In some embodiments, the gate stack structure includes additional layers, such as interfacial layers, capping layers, diffusion/barrier layers, or other applicable layers.

The gate stack structure is formed by a deposition process, a photolithography process and an etching process. The deposition process include chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), high density plasma CVD (HDPCVD), metal organic CVD (MOCVD), remote plasma CVD (RPCVD), plasma enhanced CVD (PECVD), plating, other suitable methods, and/or combinations thereof. The photolithography processes include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying (e.g., hard baking). The etching process includes a dry etching process, a wet etching process or a combinations thereof. Alternatively, the photolithography process is implemented or replaced by other proper methods such as maskless photolithography, electron-beam writing, and ion-beam writing.

FIGS. 2A-2F show side views of various stages of forming a fin field effect transistor (FinFET) device structure, in accordance with some embodiments of the disclosure. FIGS. 2A-2F show side views taken along arrow 1 of FIG. 1 and arrow 1 is parallel to the X-axis.

Referring to FIG. 2A, a first hard mask layer 112 is formed on the gate electrode 110, and a second hard mask layer 114 is formed on the first hard mask layer 112. In some embodiments, the first hard mask layer 112 is made of silicon oxide, silicon nitride, silicon oxynitride, or other applicable materials. In some embodiments, the second hard mask layer 114 is made of silicon oxide, silicon nitride, silicon oxynitride, or other applicable materials.

Gate sidewall spacers 115 are formed on the opposite sidewalls of the gate electrode 110, fin sidewall spacers 105 are formed on the opposite sidewalls of the fin structure 104. The gate sidewall spacers 115 and the fin sidewall spacers 105 independently include dielectric materials, such as silicon oxide, silicon nitride, silicon carbide (SiC), silicon oxynitride, or combinations thereof.

Afterwards, a bottom anti-reflective coating (BARC) layer 202 is formed on the gate sidewall spacers 115. The BARC layer 202 is used under a photoresist layer for enhancing pattern transfer to the hard mask layers 112, 114 during a patterning process. In some embodiments, when an implantation process is performed on N-type FinFET device structure (NMOS) 15, the BRAC 202 and a photoresist (not shown) which is formed on the BRAC 202 are formed on the gate electrode 110 to cover the gate electrode 110 in the P-type FinFET device structure (PMOS) 25.

Afterwards, the photoresist (not shown) and BRAC 202 are removed by an etching process as shown in FIG. 2B, in accordance with some embodiments of the disclosure. The etching process may be a dry etching process or a wet etching process. In some embodiments, a first dry etching process is operated at a pressure in a range from about 3 mtorr to about 50 mtorrr. In some embodiments, the gas used in the first dry etching process includes methane (CH₄), nitrogen (N₂), helium (He), oxygen (O₂) or combinations thereof. In some embodiments, the first dry etching process is operated by a power in a range from about 50 W to about 1000 W. In some embodiments, the first dry etching process is operated at a temperature in range from about 20° C. to about 80° C.

After the BRAC 202 is removed, a portion of the gate sidewall spacers 115 and a portion of the fin sidewall spacers 105 are removed as shown in FIG. 2C, in accordance with some embodiments of the disclosure. More specifically, a top portion of the gate sidewall spacers 115 is removed to expose the second hard mask layer 114. A top portion of the fin sidewall spacers 105 is removed to expose the fin structure 104.

In some embodiments, when the gate sidewall spacers 115 and the fin sidewall spacers 105 are made of silicon nitride, a second etching process is performed to remove the silicon nitride. In some embodiments, the second etching process is a second dry etching process and is operated at a pressure in a range from about 3 mtorr to about 50 mtorrr. In some embodiments, the gas used in the second dry etching process includes fluoromethane (CH₃F), difluoromethane (CH₂F₂), methane (CH₄), argon (Ar), hydrogen bromide (HBr) nitrogen (N₂), helium (He), oxygen (O₂) or combinations thereof. In some embodiments, the second dry etching process is operated by power in a range from about 50 W to about 1000 W. In some embodiments, the second dry etching process is operated at a temperature in range from about 20° C. to about 70° C.

After the second dry etching process, each of the fin sidewall spacers 105 has a first height H₁. In some embodiments, the first height H₁ is in a range from about 0.1 nm to about 50 nm.

After the portion of the gate sidewall spacers 115 and the portion of the fin sidewall spacers 105 are removed, the remaining fin sidewall spacers 105 are removed as shown in FIG. 2D, in accordance with some embodiments of the disclosure. The fin sidewall spacers 105 are removed by a third etching process. The third etching process may be a dry etching process or a wet etching process.

In some embodiments, the third etching process is a third dry etching process and is operated at a pressure in a range from about 3 mtorr to about 50 mtorrr. In some embodiments, the gas used in the third dry etching process includes fluoromethane (CH₃F), difluoromethane (CH₂F₂), methane (CH₄), argon (Ar), hydrogen bromide (HBr) nitrogen (N₂), helium (He) or oxygen (O₂) or combinations thereof. In some embodiments, the third dry etching process is operated by a power in a range from about 50 W to about 1000 W. In some embodiments, the third dry etching process is operated at a temperature in range from about 20° C. to about 70° C.

The performance of the FinFET device structure is relative to the volume of an epitaxial structure (such as 210 as shown in FIG. 2F). If the fin sidewall spacers 105 are remaining on the isolation structure, the volume an epitaxial structure (such as 210 as shown in FIG. 2F) will be limited by the fin sidewall spacers 105. In order to obtain a large volume of the epitaxial structure, it should be noted that the overall fin sidewall spacers 105 are removed. In other words, no fin sidewall spacers are formed adjacent to the fin structure 104.

After the third dry etching process, a portion of the fin structure 104 is removed as shown in FIG. 2E, in accordance with some embodiments of the disclosure. Afterwards, a portion of the isolation structure 108 is removed. The fin structure 104 and the isolation structure 108 are independently removed by an etching process, such as a dry etching process or a wet etching process.

As shown in FIG. 2E, a top surface of the remaining fin structure 104 is lower with a top surface of the isolation structure 108. A trench 204 is formed by recessing a portion of the fin structure 104 and the portion of the isolation structure 108. The recessing process includes a dry etching process, a wet etching process or combinations thereof. A wet etching solution used in the wet etching process includes tetraamethylammonium hydroxide (NH₄OH), potassium hydroxide (KOH), hydrofluoric acid (HF), tetramethylammonium hydroxide (TMAH), another application solution or combinations thereof. The dry etching process includes a biased plasma etching process. In some embodiments, the gas used in the biased plasma etching process includes tetrafluoromethane (CF4), Nitrogen trifluoride (NF₃), sulfur hexafluoride (SF₆) or helium (He).

It should be noted that the epitaxial structure (such as 210, as shown in FIG. 2F) will be formed in the trench 204, and therefore the size of the trench 204 should be well controlled. The trench 204 has a bottom surface and sloping sidewalls adjoined to the bottom surface. The trench 204 has a depth D₁ and an angle θ₁ between the bottom surface and the sidewall. In some embodiments, the depth D₁ is in a range from about 0.1 nm to about 50 nm. In some embodiments, the angle θ₁ between the bottom surface and the sidewall of the trench 204 is in a range from about 90 degrees to about 175 degrees. If the angle θ₁ is too great, the epitaxial structure (such as 210, as shown in FIG. 2F) may have too large spacing to grow. If the angle θ₁ is too small, the volume of the epitaxial structure (such as 210, as shown in FIG. 2F) will be restricted by small space, and the epitaxial structure will be smaller. The device mobility of the epitaxial structure will be affected the volume.

After the portion of the fin structure 104 and the portion of the isolation structure 108 are removed, an epitaxial structure 210 is formed on the fin structure 104 as shown in FIG. 2F, in accordance with some embodiments of the disclosure.

The epitaxial structure 210 includes source/drain epitaxial structure. In some embodiments, when an N-type FET (NFET) device is desired, the source/drain epitaxial structures include an epitaxially grown silicon (epi Si). Alternatively, when a P-type FET (PFET) device is desired, epitaxial source/drain structures include an epitaxially growing silicon germanium (SiGe). The epitaxial structure 210 may have a single layer or a multiple layers.

It should be noted that the interface between the epitaxial structure 210 and the fin structure 104 is lower than the top surface of the isolation structure 108. The epitaxial structure 210 is formed in the trench 204 and continually extends upwards to form a pentagon-like shape.

FIG. 3 is an enlarged representation of region A of FIG. 2F, in accordance with some embodiments of the disclosure. As shown in FIG. 3, the epitaxial structure 210 has the pentagon-like shape. The epitaxial structure 210 has a first surface 210A, a second surface 210B, a third surface 210C, a fourth surface 210D and a fifth surface 210E. Each of the first surface 210A, a second surface 210B, a third surface 210C, a fourth surface 210D has a (111) crystallographic orientation.

A first intersection P₁ between the first surface 210A and the second surface 210B is higher than a top surface of the isolation structure. A second intersection P₂ between the third surface 210C and the fourth surface 210D is higher than a top surface of the isolation structure. The first intersection P₁ and the second intersection P₂ are substantially in the same level. The first intersection P₁ extends from the top surface of the isolation structure 108 to a height H₂. In some embodiments, the height H₂ is in a range about 0.1 nm to about 50 nm. An angle θ₁ between the fifth surface 210E and the first surface 210A is in a range from about 90 degrees to about 175 degrees. An angle θ2 between the first surface 210A and the second surface 210B is in a range from 10 degrees to about 175 degrees.

As shown in FIG. 3, the epitaxial structure 210 has a height H₃ and a width W₁. In some embodiments, the height H₃ is in a range from about 1 nm to about 100 nm. If the height H₃ is too great, the electric resistance becomes lower. If the height H₃ is too small, electric resistance becomes higher to impact device speed. In some embodiments, the width W₁ is in a range from about 1 nm to about 100 nm. If the width W₁ is too great, the epitaxial structure 210 may merge with neighbor one and cause short circuit effect. If the width W₁ is too small, a contact window for contacting with the epitaxial structure 210 will become narrow, and therefore the circuit effect may be broken. The fin structure 104 has a width W₂. In some embodiments, the width W₂ of the fin structure 104 is smaller than width W₁ of the epitaxial structure 210.

In addition, a ratio (H₃/W₁) of the height H₃ of the epitaxial structure 210 to width W₁ of the epitaxial structure 210 is in a range from about 1 to about 100. If the ratio is too great, the EPI height will be short to affect resistance value. If the ratio is too small, the EPI volume will be smaller to reduce tension of device. Both of all will impact the mobility of device.

The epitaxial structure 210 includes a single-element semiconductor material such as germanium (Ge) or silicon (Si); or compound semiconductor materials, such as gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs); or semiconductor alloy, such as silicon germanium (SiGe), gallium arsenide phosphide (GaAsP).

The epitaxial structure 210 is formed by an epi process. The epi process may include a selective epitaxial growth (SEG) process, a chemical vapor deposition (CVD) process (e.g., vapor-phase epitaxy (VPE), a low pressure chemical vapor deposition (LPCVD) and/or ultra-high vacuum CVD (UHV-CVD)), molecular beam epitaxy, another applicable epi processes, or combinations thereof. The formation process of the epitaxial structure 210 may use gaseous and/or liquid precursors, which may interact with the composition of the fin structure 104 thereunder.

The epitaxial structure 210 may be doped or undoped in-situ during the epi process. For example, the epitaxially grown SiGe epitaxial structure may be doped with boron; and the epitaxially grown Si epitaxial structure may be doped with carbon to form a Si:C epitaxial structure, phosphorous to form a Si:P epitaxial structure, or both carbon and phosphorous to form a SiCP epitaxial structure. The doping may be achieved by an ion implantation process, plasma immersion ion implantation (PIII) process, gas and/or solid source diffusion process, another suitable process or combinations thereof. The epitaxial structure 210 may further be exposed to annealing processes, such as a rapid thermal annealing process. The annealing process is used to activate the dopants. The annealing process includes rapid thermal annealing (RTA) and/or laser annealing process.

If the epitaxial structure 210 is not doped in-situ, a second implantation process (i.e., a junction implant process) is performed to dope the epitaxial structure 210.

The fin structure 104 includes a channel region (not shown) surrounded or wrapped by the gate electrode 110. The lattice constants of the epitaxial structure 210 are different from the substrate 102, in that the channel regions are strained or stressed to enable carrier mobility of the FinFET device structure and enhance the FinFET device structure performance.

Afterwards, The FinFET device structure may continue to undergo other processes to form other structures or devices. In some embodiments, metallization includes vertical interconnects, such as conventional vias or contacts, and horizontal interconnects, such as metal lines. The various interconnection features may implement various conductive materials including copper, tungsten, and/or silicide.

The performance of the FinFET device structure is relative to the volume of the epitaxial structure 210. If the volume of the epitaxial structure 210 is too small, the operation speed of the FinFET device structure is too small to meet the requirement.

As mentioned above, if the fin sidewall spacers 105 are remaining on the isolation structure 108, the growth volume of the epitaxial structure is limited by the fin sidewall spacers 105. In order to obtain a large volume of the epitaxial structure, the fin sidewall spacers 105 are completely removed. In addition, a portion of the isolation structure 108 is removed to enlarge the width of the trench 204. It should be noted that the trench 204 is designed to have a depth D₁ and angle θ₁, and therefore the epitaxial structure 210 has more space to grow or be extended.

It should be noted that the volume and the height H₁ of the epitaxial structure 210 are controlled by adjusting the depth D₁ and angle θ₁ of the trench 204. Once the volume and the heights H₁ of the epitaxial structure 210 are controlled well, the performance of the FinFET device structure is further improved. More specifically, the operation speed of the FinFET device structure is further increased. In addition, the resistance of the gate electrode 110 may be reduced.

Embodiments for forming fin field effect transistor (FinFET) device structure are provided. The FinFET device structure includes an isolation structure formed on a substrate, and a fin structure extending above the substrate. A trench is formed by recessing a portion of the fin structure and a portion of the isolation structure. An epitaxial structure is formed on the fin structure and in the trench. The epitaxial structure is adjacent to the gate stack structure. The volume and the height of the epitaxial structure are controlled by adjusting a depth and an angle of a trench. Once the volume of the epitaxial structure is efficiently controlled, the performance of the FinFET device structure is further improved. More specifically, the operation speed of the FinFET device structure is further increased.

In some embodiments, a fin field effect transistor (FinFET) device structure is provided. The FinFET structure includes a substrate and an isolation structure formed on the substrate. The FinFET structure also includes a fin structure extending above the substrate, and the fin structure is embedded in the isolation structure. The FinFET structure further includes an epitaxial structure formed on the fin structure, the epitaxial structure has a pentagon-like shape, and the interface between the epitaxial structure and the fin structure is lower than the top surface of the isolation structure.

In some embodiments, a fin field effect transistor (FinFET) device structure is provided. The FinFET structure includes a substrate and an isolation structure formed on the substrate. The FinFET structure also includes a fin structure extending above the substrate, and the fin structure protrudes from the isolation structure. The FinFET structure further includes an epitaxial structure formed on the fin structure, the epitaxial structure comprises a bottom surface and a first surface adjoined to the bottom surface, and an angle between the bottom surface and the first surface is in a range from about 90 degrees to about 175 degrees.

In some embodiments, a method for forming a fin field effect transistor (FinFET) device structure is provided. The method includes providing a substrate and forming an isolation structure on the substrate. The method also includes forming a fin structure above the substrate, and the fin structure is embedded in the isolation structure. The method further includes forming fin sidewall spacers on a top surface and sidewalls the fin structure and removing the fin sidewall spacers to expose the fin structure. The method includes recessing a portion of the fin structure and a portion of the isolation structure to form a trench in the isolation structure. The method further includes epitaxially growing an epitaxial structure from the trench, the epitaxial structure is formed over the fin structure, and an interface between the epitaxial structure and the fin structure is lower than a top surface of the isolation structure.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

1. A fin field effect transistor (FinFET) device structure, comprising: a substrate; an isolation structure formed on the substrate; a fin structure extending above the substrate, wherein the fin structure is embedded in the isolation structure; and an epitaxial structure formed on a first portion of the fin structure, wherein a cross section of the epitaxial structure has a pentagon-like shape, an interface between the epitaxial structure and the first portion of fin structure is lower than a top surface of the isolation structure, a top of the first portion of the fin structure is lower than the top surface of the isolation structure, and a width of a bottom side of the pentagon-like shape is smaller than a width of the pentagon-like shape at a location level with the top surface of the isolation structure.
 2. The fin field effect transistor (FinFET) device structure as claimed in claim 1, further comprising: a gate stack structure is formed over a central portion of the fin structure; and gate sidewalls formed adjacent to the gate stack structure.
 3. The fin field effect transistor (FinFET) device structure as claimed in claim 1, wherein the epitaxial structure is formed in a trench in the isolation structure, and the trench has a depth in a range from about 0.1 nm to about 50 nm.
 4. The fin field effect transistor (FinFET) device structure as claimed in claim 1, wherein the epitaxial structure comprises a bottom surface and a first surface adjoined to the bottom surface, and wherein an angle between the bottom surface and the first surface is in a range from about 90 degrees to about 175 degrees.
 5. The fin field effect transistor (FinFET) device structure as claimed in claim 4, wherein the epitaxial structure further comprises a second surface adjoined to the first surface, and an intersection between the first surface and the second surface is higher than the top surface of the isolation structure.
 6. The fin field effect transistor (FinFET) device structure as claimed in claim 5, wherein an angle between the first surface and the second surface is about 1 degree to about 175 degrees.
 7. The fin field effect transistor (FinFET) device structure as claimed in claim 1, wherein the epitaxial structure comprises a source/drain structure.
 8. The fin field effect transistor (FinFET) device structure as claimed in claim 1, wherein the epitaxial structure comprises a first width, the fin structure has a second width, and the first width is greater than the second width.
 9. The fin field effect transistor (FinFET) device structure as claimed in claim 8, wherein the epitaxial structure comprises a first height, and a ratio of the first height to the first width is in a range from about 1 to about
 100. 10. A fin field effect transistor (FinFET) device structure, comprising: a substrate; an isolation structure formed on the substrate; a fin structure extending above the substrate, wherein the fin structure protrudes from the isolation structure and has a first portion embedded in the isolation structure; and an epitaxial structure formed directly above the first portion of the fin structure, wherein the epitaxial structure comprises a bottom surface and a first surface adjoined to the bottom surface, an angle between the bottom surface and the first surface is in a range from about 90 degrees to about 175 degrees, and a top of the first portion of the fin structure is lower than the top surface of the isolation structure.
 11. The fin field effect transistor (FinFET) device structure as claimed in claim 10, wherein the epitaxial structure further comprises a second surface adjoined to the first surface, and an intersection between the first surface and the second surface is higher than the top surface of the isolation structure.
 12. The fin field effect transistor (FinFET) device structure as claimed in claim 10, wherein the epitaxial structure has a first width and a first height, and a ratio of the first height to the first width is in a range from about 1 to about
 100. 13. The fin field effect transistor (FinFET) device structure as claimed in claim 11, wherein the fin structure has a second width, and the first width is greater than the second width.
 14. The fin field effect transistor (FinFET) device structure as claimed in claim 10, further comprising: a gate stack structure formed over a central portion of the fin structure; and the epitaxial structure formed adjacent to the gate stack structure.
 15. The fin field effect transistor (FinFET) device structure as claimed in claim 13, wherein the epitaxial structure is formed in a trench in the isolation structure, and the trench has a depth in a range from about 0.1 nm to about 50 nm.
 16. The fin field effect transistor (FinFET) device structure as claimed in claim 10, wherein the epitaxial structure comprises a source/drain structure. 17-20. (canceled)
 21. The fin field effect transistor (FinFET) device structure as claimed in claim 1, wherein no fin sidewall spacers are formed adjacent to the fin structure.
 22. The fin field effect transistor (FinFET) device structure as claimed in claim 1, wherein the lattice constants of the epitaxial structure are different from that of the substrate.
 23. The fin field effect transistor (FinFET) device structure as claimed in claim 5, wherein the intersection has a second height above the top surface of the isolation structure.
 24. The fin field effect transistor (FinFET) device structure as claimed in claim 23, wherein the second height is in a range about 0.1 nm to about 50 nm. 